High strength hot rolled thick steel sheet excellent in strength and toughness after heat treatment and method for manufacturing the same

ABSTRACT

A high strength hot rolled thick steel sheet has a tensile strength of 440 to 640 MPa, preferably 490 to 590 MPa, and an elongation of 20% or more; is excellent in uniformity in a sheet thickness direction and in strength and toughness after heat treatment; and is suitably used for structural components of automobiles, construction machines, and the like, and a method for manufacturing the hot rolled thick steel sheet. A steel material having a composition including C: 0.10 to 0.20%, Ti: 0.01 to 0.15%, B: 0.0010 to 0.0050%, and proper amounts of Si, Mn, Al, P, S, and N is hot-rolled at a finisher delivery temperature of 820 to 880° C. in finish rolling, is cooled at a cooling rate of 15 to 50° C./s until a temperature reaches a cooling stop temperature of 500 to 600° C., and is coiled.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/060805, withan international filing date of Jun. 6, 2008 (WO 2009/004909 A1,published Jan. 8, 2009), which is based on Japanese Patent ApplicationNo. 2007-171898, filed Jun. 29, 2007, the subject matter of which isincorporated by reference.

TECHNICAL FIELD

This disclosure relates to a hot rolled thick steel sheet and a methodfor manufacturing the steel sheet. “A hot rolled thick steel sheet”herein is a hot rolled steel sheet having a sheet thickness of 6 mm ormore and 12 mm or less, which is a relatively thick hot rolled steelsheet. Such a hot rolled thick steel sheet is suitably used as amaterial for manufacturing structural components of, for example,automobiles and construction equipment (hereinafter also, referred to as“construction machines”).

BACKGROUND

In recent years, regulations of emissions limit law for automobiles havebeen tightened in terms of global environmental protection and weightreduction of a car body has been promoted to improve fuel economy.Automobile components are also not exceptions and weight reduction ofautomobile components has been strongly demanded. Similarly, weightreduction of structural components of construction machines or the likehas been also strongly demanded. This is because large, heavy, andthick-walled materials having a sheet thickness of about 6 mm or moreand 12 mm or less and a length of 10 m are often used for the structuralcomponents of automobiles, construction machines, and the like. If ahighly strengthened steel sheet is used to reduce the weight ofcomponents, the formability of a steel sheet such as elongation isdecreased, which poses a problem in that the degree of difficulty inprocessing is considerably increased. In addition; there is a problem inthat fatigue strength is not improved at stress concentration zones suchas holes opened for weight reduction and weld zones that inevitablyexist. Therefore, unlike other small thin-walled parts, largethick-walled parts such as structural components of automobiles,construction machines, and the like had a tensile strength of at mostabout 540 MPa even after being strengthened.

In recent years, die quench, in which parts are quenched while beingpressed, has been put to practical use as a means for strengtheningsmall thin-walled parts. However, when die quench is applied to largethick-walled parts, there are various problems in that huge equipmentneeds to be prepared, desired strength cannot be achieved because partsare not quenched to their center due to their thick wall, and thebrittle failure unique to thick-walled parts is caused when the partsare as quenched. Thus, die quench is unsuitable for large thick-walledparts.

However, weight reduction of structural components of automobiles,construction machines, and the like has been strongly demanded andstrengthening of components has been desired. Therefore, particularlyfor components for which high strength is demanded, a material isprocessed into a shape of components and heat treatment such asquenching and tempering is then performed to achieve high strength andhigh toughness of components. Thus, in addition to strength andelongation, excellent component strength and toughness achieved by heattreatment performed after a material is processed into a shape ofcomponents have been demanded for a hot rolled steel sheet that is a rawmaterial.

To meet such a demand, for example, Japanese Unexamined PatentApplication Publication No. 2002-309344 discloses a method formanufacturing a thin steel sheet including a step of hot-rolling a steelmaterial at a coiling temperature of 720° C. or less, the steel materialcontaining C: 0.10 to 0.37% and proper amounts of Si, Mn, P, S, and Aland containing B and N so as to satisfy 14B/10.8N: 0.50 or more, whereinBN that is an intrasteel precipitate has an average grain size of 0.1 μmor more, and prior austenite after quenching has a grain size of 2 to 25μm. According to the technology described in Japanese Unexamined PatentApplication Publication No. 2002-309344, a thin steel sheet havingexcellent hardenability at a low temperature for a short time afterprocessing, excellent toughness after quenching, and little variation ofcharacteristics according to quenching conditions can be manufactured.

Japanese Unexamined Patent Application Publication No. 2002-309345discloses a method for manufacturing a thin steel sheet that isexcellent in toughness for impact after quenching, the method includinga step of hot-rolling a steel material at a coiling temperature of 720°C. or less, the steel material containing C: 0.10 to 0.37% and properamounts of Si, Mn, P, S, Al, and Ti and containing B and N so as tosatisfy effective B amount: 0.0005% or more, wherein TiN that is anintrasteel precipitate has an average grain size of 0.06 to 0.30 μm, andprior austenite after quenching has a grain size of 2 to 25 μm.According to the technology described in Japanese Unexamined PatentApplication Publication No. 2002-309345, a thin steel sheet havingexcellent hardenability at a low temperature for a short time afterprocessing, excellent toughness for impact after quenching, and littlevariation of characteristics according to quenching conditions can bemanufactured.

However, the technologies described in Japanese Unexamined PatentApplication Publication Nos. 2002-309344 and 2002-309345 focus on arelatively thin hot rolled steel sheet having a thickness of about 2.4mm. When the technologies described in these publications are applied tomanufacturing of a relatively thick hot rolled steel sheet used forlarge thick-walled parts such as structural components of automobiles,construction machines, and the like, the microstructure changes in itssheet thickness direction and the strength and ductility are decreasedin the center in the sheet thickness direction. Therefore, a hot rolledsteel sheet having a uniform microstructure in the sheet thicknessdirection and desired strength and ductility cannot be obtained.Furthermore, desired strength and toughness after heat treatment cannotbe achieved.

To obtain a desired uniform microstructure in the center in the sheetthickness direction, a hot rolled thick steel sheet used for largethick-walled parts such as structural components of automobiles,construction machines, and the like needs to be quenched after hotrolling. However, quenching after hot rolling causes the cooling rate atan outer layer of the steel sheet (particularly around the edges in asheet width direction) to become too high, which causes martensitictransformation. As a result, the outer layer of the steel sheet ishardened, and a hot rolled steel sheet partially having large deviationof hardness along thickness is obtained. When such a hot rolled steelsheet (coil) is cut into raw materials for components, inhomogeneousdeformation (called a camber when a hot rolled steel sheet is slit inthe width direction) is caused and the dimensional accuracy of the cutmaterials is decreased. Consequently, the dimensional accuracy ofcomponents is decreased.

It could therefore be helpful to provide a high strength hot rolledthick steel sheet that is excellent in strength and toughness after heattreatment; that has a tensile strength of 440 to 640 MPa, preferably 490to 590 MPa, and an elongation of 20% or more (gauge length GL: 50 mm)required for large thick-walled parts; and whose deviation of hardnessalong thickness is within 10% from the average. It could also be helpfulto provide a method for manufacturing the hot rolled thick steel sheet.

Summary

We thus provide:

-   -   (1) A high strength hot rolled thick steel sheet with a sheet        thickness of 6 mm or more and 12 mm or less that is excellent in        strength and toughness after heat treatment includes a        composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn:        0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to        0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to        0.0050% by mass with the balance Fe and incidental impurities;        and a bainitic ferrite phase having an area ratio of 95% or        more, wherein a deviation of hardness along thickness is within        10% from an average; and a tensile strength of 440 to 640 MPa        and an elongation of 20% or more (gauge length GL: 50 mm) are        satisfied.    -   (2) A method for manufacturing a high strength hot rolled thick        steel sheet that is excellent in strength and toughness after        heat treatment includes the steps of hot-rolling a steel        material at a finisher delivery temperature of 820 to 880° C. in        finish rolling to obtain a hot rolled steel sheet having a sheet        thickness of 6 mm or more and 12 mm or less, the steel material        having a composition including C: 0.10 to 0.20%, Si: 0.01 to        1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al:        0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B:        0.0010 to 0.0050% by mass with the balance Fe and incidental        impurities; cooling the hot rolled steel sheet at a cooling rate        of 15 to 50° C./s on a surface temperature basis until a surface        temperature reaches a temperature range of 550 to 650° C.; and        coiling the hot rolled steel sheet in the temperature range,        wherein a deviation of hardness along thickness is within 10%        from an average; and a tensile strength of 440 to 640 MPa and an        elongation of 20% or more (gauge length GL: 50 mm) are        satisfied.

A hot rolled thick steel sheet with a sheet thickness of 6 mm or moreand 12 mm or less that has desired high strength and excellentformability, specifically a tensile strength of 440 to 640 MPa and anelongation of 20% or more, and that has uniform hardness distribution ina sheet thickness direction, specifically whose deviation of hardnessalong thickness is within 10% from the average, can be manufacturedeasily and stably. This produces industrially significant effects.Furthermore, the hot rolled steel sheet is excellent in strength andtoughness after heat treatment. Therefore, large thick-walled parts(products) having high strength, high ductility, and high toughness suchas structural components of automobiles, construction machines, and thelike can be manufactured easily and stably by processing a hot rolledsteel sheet into a desired shape and then performing heat treatment.

DETAILED DESCRIPTION

A hot rolled thick steel sheet that is “excellent in strength andtoughness after heat treatment” herein is a hot rolled steel sheethaving high strength and high ductility, specifically a tensile strengthof 980 MPa or more and an elongation of 15% or more (GL: 50 mm) in atypical water quenching and tempering treatment (about 930° C. heatingwater quenching-about 200° C. tempering); and having a high toughness,specifically a ductile-brittle fracture transition temperature vTrs of−60° C. or less in a Charpy impact test.

The heat treatment conditions applied to components composed of thesteel sheet are not limited to the above-described typical waterquenching and tempering treatment (about 930° C. heating waterquenching-about 200° C. tempering). For example, desired heat treatmentconditions such as about 930° C. heating water quenching-about 400° C.tempering can be used.

We considered the factors that affect the strength and formability(ductility) of a relatively thick hot rolled steel sheet having a sheetthickness of 6 mm or more and 12 mm or less and also the factors thataffect the strength and toughness after heat treatment. Consequently, wefound that, with a composition including proper amounts of Ti and B in alow-carbon steel with C: 0.10 to 0.20% by mass and a low N content of0.005% by mass and with a bainitic ferrite single phase that is auniform microstructure across the entire thickness, the deviation ofhardness along thickness comes within 10% from the average and themicrostructure after heat treatment becomes uniform martensite acrossthe entire thickness while desired high strength and excellentformability are achieved, whereby a hot rolled thick steel sheet that isexcellent in strength and toughness after heat treatment can beobtained. We also found that, by adjusting a cooling rate after hotrolling to 15 to 50° C./s on a surface temperature basis, themicrostructure can form a bainitic ferrite single phase that is uniformacross the entire thickness, whereby the deviation of hardness alongthickness comes within 10% from the average.

Because the hot rolled steel sheet is mainly used for large structuralcomponents of automobiles, construction machines, and the like, thesheet thickness is limited to 6 mm or more and 12 mm or less.

The reason for limiting the composition of the hot rolled steel sheetwill be described first. Hereinafter, % by mass is simply expressed as%.

C: 0.10 to 0.20%

C is an element that forms a carbide in a steel and effectivelycontributes to an increase in the strength of a steel sheet. Inquenching treatment, C is an element that facilitates martensitictransformation and effectively contributes to strengthening of amicrostructure caused by a martensitic phase. A C content of 0.10% ormore is necessary. When the C content is less than 0.10%, it isdifficult to achieve desired sheet strength (tensile strength: 440 MPaor more) and desired strength after heat treatment (tensile strength:980 MPa or more). On the other hand, when the C content is more than0.20%, the sheet strength and the strength after heat treatment becometoo high, which reduces formability and toughness, thereby decreasingweldability. Thus, the C content is limited to 0.10 to 0.20%. Si: 0.01to 1.0%

Si is an element that effectively contributes to an increase in thestrength of steel through solution hardening. A Si content of 0.01% ormore is necessary to produce such an effect. On the other hand, when theSi content is more than 1.0%, unevenness called a red scale is formed ona surface and surface properties are degraded. This decreases elongationand fatigue strength. Thus, the Si content is limited to 0.01 to 1.0%.Preferably, the Si content is 0.35% or less.

Mn: 0.5 to 2.0%

Mn is an element that effectively contributes to an increase in thestrength of steel through solution hardening and an increase in thestrength of steel through the improvement in hardenability. A Mn contentof 0.5% or more is necessary to produce such an effect. On the otherhand, when the Mn content is more than 2.0%, segregation appearsmarkedly and it is difficult to form a bainitic ferrite single phaseacross the entire thickness. Consequently, the characteristics of asteel sheet and the quality of a material after heat treatment aredegraded. Thus, the Mn content is limited to 0.5 to 2.0%. Preferably,the Mn content is 1.0 to 2.0%.

P: 0.03% or less

P increases the strength of steel through solution hardening, butproduces segregation and decreases the uniformity of the quality of amaterial, thereby significantly decreasing toughness after heattreatment. Therefore, the P content is preferably reduced as much aspossible, but excess reduction increases material costs. When the Pcontent is more than 0.03%, segregation appears markedly. Thus, the Pcontent is limited to 0.03% or less. Preferably, the P content is 0.02%or less.

S: 0.01% or less

S is present as a sulfide in steel and decreases ductility, therebyreducing bending workability and the like. Therefore, the S content ispreferably reduced as much as possible, but excess reduction increasesmaterial costs. When the S content is more than 0.01%, toughness afterheat treatment is significantly reduced. Thus, the S content is limitedto 0.01% or less. Preferably, the S content is 0.005% or less.

Al: 0.01 to 0.10%

Al is an element that functions as a deoxidizer. Such an effect markedlyappears when an Al content is 0.01% or more. However, an Al content ofmore than 0.1% decreases formability and hardenability. Thus, the Alcontent is limited to 0.01 to 0.1%. Preferably, the Al content is 0.05%or less.

N: 0.005% or less

N decreases formability by forming nitrides such as TiN and AlN insteel. N also reduces the amount of B solid solution that is effectivefor improving hardenability by forming BN during quenching. Such anadverse effect of N is permissible when the N content is 0.005% or less.Thus, the N content is limited to 0.005% or less.

Ti: 0.01 to 0.15%

Ti is an element that effectively contributes to allowing amicrostructure after hot rolling to be constituted by bainitic ferriteand that contributes to producing an effect of improving hardenabilitythrough a B solid solution because Ti forms a nitride prior to B. Sucheffects are produced when a Ti content is 0.01% or more. However, a Ticontent of more than 0.15% increases deformation resistance during hotrolling and excessively increases rolling load, thereby decreasingtoughness after heat treatment. Thus, the Ti content is limited to 0.01to 0.15%. Preferably, the Ti content is 0.03 to 0.10%.

B: 0.0010 to 0.0050%

B is an element that suppresses the formation of polygonal ferrite andpearlite during cooling performed after hot rolling and that effectivelycontributes to improving hardenability and toughness during heattreatment. In the case where a thick steel sheet having a thickness of 6mm or more is used, such effects markedly appear when a B content is0.0010% or more. On the other hand, a B content of more than 0.0050%increases deformation resistance during hot rolling and excessivelyincreases rolling load. In addition, such a B content forms bainite andmartensite after hot rolling and poses a problem such as sheet cracking.Thus, the B content is limited to 0.0010 to 0.0050%. Preferably, the Bcontent is 0.0015 to 0.0040%.

The balance other than the components described above is Fe andincidental impurities. For example, Cu: 0.3% or less and Cr: 0.3% orless are permissible as incidental impurities.

The hot rolled thick steel sheet has the above-described composition anda bainitic ferrite single phase across the entire thickness. A singlephase herein is constituted by a bainitic ferrite phase having an arearatio of 95% or more. A bainitic ferrite phase includes needle-shapedferrite and acicular ferrite. Note that 5% or less of a polygonalferrite phase, a pearlite phase, a cementite phase, a bainite phase, amartensite phase, and the like on an area ratio basis are permissible asa microstructure other than the bainitic ferrite phase.

By forming a bainitic ferrite single phase across the entire thickness,a hot rolled thick steel sheet can be provided that has desired highstrength and high ductility, specifically a tensile strength of 440 MPaor more and 640 MPa or less and an elongation of 20% or more (GL: 50mm), that is excellent in formability such as a flexural property, andthat can be processed into large thick-walled parts such as structuralcomponents of automobiles, construction machines, and the like. When thearea ratio of the bainitic ferrite phase is less than 95%, both thedesired high strength and high ductility cannot be achieved. When thephase fraction of the bainitic ferrite phase is decreased to less than95%, the uniformity of the microstructure is reduced. As a result,cambering or the like is caused when cutting and the dimensionalaccuracy is reduced, thereby decreasing formability such as a flexuralproperty. To judge whether a bainitic ferrite single phase is formedacross the entire thickness, the area ratios of a bainitic ferrite phaseare obtained at a depth of 0.1 mm from the surface, at a position of aquarter the way through the sheet thickness, and at a position of a halfthe way through the sheet thickness. When the area ratios are 95% ormore at all of the three positions, it is judged that a bainitic ferritesingle phase is formed across the entire thickness.

A preferable method for manufacturing a hot rolled thick steel sheetwill now be described.

A molten steel having the above-described composition is preferablysmelted by a typical smelting method using a converter, a vacuum meltingfurnace, or the like to make a steel material such as a slab through atypical casting method such as continuous casting or an ingotmaking-blooming method. However, the method for making a steel materialis not limited to this example, and any typical method for making asteel material can be suitably applied.

A steel material having the above-described composition is hot-rolled toobtain a hot rolled thick steel sheet having a sheet thickness of 6 mmor more and 12 mm or less. When the sheet thickness is more than 12 mm,a sufficient reduction ratio is not achieved in hot rolling and themicrostructure is coarsened after the hot rolling, which tends toproduce martensite during cooling. Thus, the sheet thickness ispreferably 12 mm or less. The heating temperature for hot rolling is notparticularly limited, and a finisher delivery temperature in hot rollingdescribed below needs only to be ensured. The heating temperature ispreferably 1000 to 1300° C., which is a typical heating temperature.When the heating temperature is more than 1300° C., crystal grains arecoarsened and hot formability is easily decreased. On the other hand,when the heating temperature is less than 1000° C., deformationresistance is excessively increased and a burden on rolling equipment isincreased, which easily poses a problem such as a difficulty in rolling.In addition, when the heating temperature is less than 1000° C., TiCthat is present in a steel material is insufficiently melted, whicheasily causes a difficulty in achieving a desired microstructure anddesired strength after hot rolling.

In the hot rolling, the finisher delivery temperature of finish rollingis 820 to 880° C.

When the finisher delivery temperature of finish rolling is 820° C. ormore, ferrite transformation is suppressed in the following coolingstep. As a result, a bainitic ferrite phase (bainitic ferrite singlephase) having an area ratio of 95% or more can be formed. When thefinisher delivery temperature of finish rolling is less than 820° C.,ferrite transformation is facilitated in the following cooling step. Asa result, a bainitic ferrite single phase is not easily formed. On theother hand, the finisher delivery temperature of finish rolling is morethan 880° C., not only ferrite transformation but also bainitic ferritetransformation is suppressed. As a result, a bainitic ferrite singlephase is not easily formed and a bainite phase and a martensite phaseare easily formed. The formation of a bainite phase and a martensitephase may excessively increase the strength of a steel sheet and causecracking on a steel sheet in coiling or rewinding, of a coil. For thisreason, the finisher delivery temperature of finish rolling is limitedto 820 to 880° C.

After the completion of rolling, the hot rolled steel sheet is cooled ata cooling rate of 15 to 50° C./s on a sheet surface temperature basisuntil a surface temperature reaches a temperature range of 550 to 650°C.

To form a bainitic ferrite single phase across the entire thickness of asteel sheet, a cooling rate is adjusted so as to be 15° C./s or more ona sheet surface temperature basis in the cooling performed after thecompletion of rolling. When the cooling rate is less than 15° C./s on asurface temperature basis, a polygonal ferrite phase is easilyprecipitated, for example, in the center in a sheet thickness direction,which makes it difficult to form a uniform bainitic ferrite single phasein a sheet thickness direction. On the other hand, when the cooling rateis more than 50° C./s on a surface temperature basis, martensite isproduced on an outer layer and a uniform bainitic ferrite single phasecannot be formed in a sheet thickness direction. Consequently, thedeviation of hardness along thickness becomes significant and it isdifficult to adjust the deviation of hardness along thickness to bewithin 10% from the arithmetic mean hardness (average) in a sheetthickness direction. In the cooling, water cooling is adopted. Thecooling rate is preferably adjusted by changing the amount and time ofwater injection. For this reason, in the cooling performed after thecompletion of rolling, the cooling rate is adjusted to 15 to 50° C./s ona sheet surface temperature basis. The above-described cooling rate on asurface temperature basis is an average value of actually measuredsurface temperatures between the finisher delivery temperature of finishrolling and the cooling stop temperature.

The above-described cooling stop temperature is in a temperature rangein which the surface temperature of a steel sheet is 550 to 650° C. Whenthe cooling stop temperature is less than 550° C. on a surfacetemperature basis, a bainite phase and a martensite phase are producedand a bainitic ferrite single phase cannot be formed. Furthermore,cracking is caused on a hot rolled steel sheet during coiling and theformability of a steel sheet is decreased due to too high strength. Onthe other hand, when the cooling stop temperature is more than 650° C.,a polygonal ferrite phase and a pearlite phase are produced and abainitic ferrite single phase cannot be formed. In addition, thestrength of a steel sheet may fall short of desired strength. Thus, thecooling stop temperature after the completion of rolling is limited to atemperature range of 550 to 650° C.

After the cooling is stopped, the hot rolled steel sheet is coiled inthe temperature range. When the coiling temperature is less than 550°C., a bainite phase and a martensite phase are produced and a bainiticferrite single phase cannot be formed. On the other hand, when thecoiling temperature is more than 650° C., a polygonal ferrite phase anda pearlite phase are produced and a bainitic ferrite single phase cannotbe formed. Consequently, the desired strength of a steel sheet cannot beachieved and the uniformity in a sheet thickness direction is decreased.Thus, the coiling temperature is limited to a temperature range of 550to 650° C. on a sheet surface temperature basis.

Example

After a steel material (steel slab) having a composition shown in Table1 was heated to heating temperature shown in Table 2, it was hot-rolledunder the finish rolling conditions shown in Table 2 to obtain a hotrolled steel sheet having a sheet thickness shown in Table 2. After thecompletion of finish rolling, the hot rolled steel sheet was cooledunder the conditions shown in Table 2 and coiled at a coilingtemperature shown in Table 2.

The obtained hot rolled steel sheet was evaluated for strength,ductility, the uniformity of hardness in a sheet thickness direction,and formability (bending workability) by performing a microstructureobservation, a tensile test, a hardness test, and a bending test.Furthermore, after a test panel was prepared from the obtained hotrolled steel sheet and then pickled to remove scales on the steel sheetsurface, heat treatment (quenching-tempering treatment) was performed.The test panel was evaluated for strength, ductility, and toughnessafter heat treatment by performing a microstructure observation, atensile test, and an impact test. The heat treatment was constituted byquenching and tempering. In the quenching treatment, the test panel washeated to 930° C. and held for 10 minutes, and then quenched in water at20° C. In the tempering treatment, the test panel was heated to 200° C.and held for 60 minutes, and then cooled in the air. After the cooling,a test piece was prepared from the test panel to perform the tests. Thetest methods are as follows.

(1) Microstructure Observation

After a test piece for microstructure observation was prepared from theobtained hot rolled steel sheet, sheet sections that were parallel tothe rolling direction of the test piece were polished and corroded withnital. The metal microstructure was observed (the number of fields ofview: 10 spots each) and imaged using a scanning electron microscope(SEM) (magnify-cation: 3000 times) at a depth of 0.1 mm from thesurface, at a position of a quarter the way through the sheet thickness,and at a position of a half the way through the sheet thickness. Thekinds of phases and the phase fraction (area ratio) of each phase weremeasured using an image analysis apparatus. The area ratio of a bainiticferrite phase was calculated by averaging measured values of 10 observedfields. When the area ratios (averages of the measured values in 10fields) of a bainitic ferrite phase measured at a depth of 0.1 mm fromthe surface, at a position of a quarter the way through the sheetthickness, and at a position of a half the way through the sheetthickness were all 95% or more, it was judged that a bainitic ferritephase having an area ratio of 95% or more (bainitic ferrite singlephase) was formed across the entire thickness.

(2) Tensile Test

A JIS No. 5 test piece (GL: 50 mm) was prepared from the obtained hotrolled steel sheet (or the test panel) such that the pulling directionwas perpendicular to the rolling direction. A tensile test was performedin conformity to JIS Z 2241. Tensile characteristics (yield strength YS,tensile strength TS, and elongation El) were obtained to evaluatestrength and ductility.

(3) Hardness Test

A test piece for hardness measurement was prepared from the obtained hotrolled steel sheet, and sheet sections that were parallel to the rollingdirection of the test piece were then polished. Vickers hardness HV(load: 9.8 N=1 kgf) was measured with a 0.2 mm pitch. The hardnessmeasurement was started at a position of 0.2 mm from a surface. When apoint to be measured next reached a position within 0.2 mm from anothersurface, the point was not measured and the hardness measurement wasfinished. The average hardness (average value) HV_(mean) of the hotrolled steel sheet was calculated by averaging the obtained hardnessvalues in the sheet thickness direction using an arithmetic mean. Inaddition, the difference ΔHV between the maximum hardness and theminimum hardness was calculated to obtain [ΔHV/HV_(mean)]×100(%). Thus,the uniformity in the sheet thickness direction was evaluated.

(4) Bending Test

A test piece for a bending test (size: sheet thickness t×100×200 mm) wasprepared from the obtained hot rolled steel sheet such that a directionperpendicular to the rolling direction was a longitudinal direction ofthe test piece. To measure the minimum bend radius (mm) that does notcause cracking on the outer side of the bent portion, 180 degree bendingwas performed at various bend radii such as bend radii of 0.5 times, 1.0time, 1.5 times, and 2.0 times the sheet thickness such that thelongitudinal direction of the test piece was a circumferentialdirection. The minimum bend radius was expressed as a ratio to the sheetthickness of the test piece.

(5) Impact Test

A V-notch test piece was prepared from the obtained test panel inconformity to JIS Z 2242 such that the longitudinal direction of thetest piece was perpendicular to the rolling direction. A Charpy impacttest was performed to obtain a ductile-brittle fracture transitiontemperature vTrs (° C.), which is a temperature at which percent ductilefracture is 50%. Thus, the toughness after heat treatment was evaluated.

Table 3 shows the obtained results.

TABLE 1 Chemical composition (% by mass) Steel Nos. C Si Mn P S Al N TiB A 0.10 0.03 1.35 0.015 0.004 0.038 0.0035 0.042 0.0018 B 0.12 0.150.83 0.013 0.003 0.042 0.0036 0.035 0.0022 C 0.15 0.03 1.24 0.010 0.0030.047 0.0042 0.038 0.0016 D 0.16 0.05 1.11 0.013 0.003 0.042 0.00400.033 0.0031 E 0.15 1.20 0.71 0.011 0.003 0.033 0.0043 0.045 0.0014 F0.15 0.03 0.25 0.024 0.004 0.044 0.0047 0.041 0.0013 G 0.15 0.03 2.340.013 0.005 0.046 0.0038 0.039 0.0016 H 0.14 0.03 0.84 0.045 0.003 0.0390.0032 0.037 0.0015 I 0.15 0.05 0.83 0.015 0.012 0.041 0.0041 0.0480.0019 J 0.16 0.03 0.81 0.012 0.003 0.043 0.0039 0.004 0.0021 K 0.150.04 0.89 0.013 0.003 0.046 0.0042 0.16  0.0014 L 0.16 0.03 0.76 0.0120.004 0.039 0.0044 0.038 0.0003 M 0.15 0.03 0.82 0.011 0.002 0.0440.0042 0.042 0.0075 N 0.16 0.70 1.24 0.015 0.003 0.047 0.0046 0.0520.0018 O 0.18 0.03 0.75 0.016 0.002 0.038 0.0042 0.043 0.0016 P 0.200.01 0.88 0.018 0.004 0.045 0.0038 0.044 0.0018 Q 0.23 0.02 0.95 0.0120.003 0.044 0.0036 0.041 0.0019 R 0.08 0.03 0.77 0.011 0.004 0.0430.0042 0.042 0.0023

TABLE 2 Hot rolling conditions Finisher delivery temperature SteelHeating of finish Cooling Cooling stop Coiling Sheet panel Steeltemperature rolling* rate* temperature* temperature* thickness Nos. Nos.(° C.) (° C.) (° C./s) (° C.) (° C.) (mm) Remarks 1 A 1200 860 40 640610 6.0 IE 2 B 1200 855 50 620 590 7.0 IE 3 C 1250 860 30 620 600 8.0 IE4 C 1250 800 40 630 600 8.0 CE 5 C 1250 920 40 610 580 8.0 CE 6 C 1250860  5 630 620 8.0 CE 7 C 1250 850 100  600 570 8.0 CE 8 C 1250 855 40550 500 8.0 CE 9 C 1250 860 45 650 680 8.0 CE 10 C 1250 870 30 690 6408.0 CE 11 C 1250 870 30 530 560 8.0 CE 12 D 1250 860 15 600 570 7.0 IE13 E 1250 860 40 630 600 8.0 CE 14 F 1250 860 40 620 590 8.0 CE 15 G1250 865 40 600 580 8.0 CE 16 H 1250 845 45 630 600 8.0 CE 17 I 1250 85040 630 610 8.0 CE 18 J 1250 860 40 640 610 8.0 CE 19 K 1250 850 40 620600 8.0 CE 20 L 1250 855 35 630 600 8.0 CE 21 M 1250 840 40 620 600 8.0CE 22 N 1250 860 40 550 550 12.0 IE 23 O 1250 855 20 650 650 10.0 IE 24P 1250 830 40 640 620 8.0 IE 25 Q 1250 860 40 640 620 8.0 CE 26 R 1200850 45 620 600 8.0 CE *on a surface temperature basis IE: InventionExample CE: Comparative Example

TABLE 3 Microstructure of hot rolled steel sheet Basic materialcharacteristics of hot rolled steel sheet Thin 0.1 mm from Position ofPosition of Tensile Hardness steel surface ¼ t ½ t characteristics HVpanel Steel BF area BF area BF area YS TS El Maximum Minimum Nos. Nos.Kind* ratio (%) Kind* ratio (%) Kind* ratio (%) (MPa) (MPa) (%) hardnesshardness Average  1 A BF 100 BF 100 BF + F  96 375 485 31 168 158 165  2B BF 100 BF 100 BF + F  98 408 516 29 174 166 171  3 C BF 100 BF 100 BF100 447 568 26 187 175 182  4 C BF + F  85 BF + F  80 BF + F  80 410 52018 176 152 163  5 C BF + M  70 BF + M  90 BF + M  90 496 645 17 232 194213  6 C BF + F  90 BF + F  85 BF + F  60 414 519 19 165 140 160  7 CBF + M  70 BF + M  90 BF + B  85 500 642 17 227 193 210  8 C BF + B + M 70 BF + B  70 BF + B  80 548 703 15 236 210 220  9 C BF + F  80 BF + F 70 BF + F  60 338 476 18 160 140 158 10 C BF + F  92 BF + F  85 BF + F 80 327 473 19 168 150 160 11 C BF + B + M  65 BF + B  80 BF + B  90 554690 14 260 220 224 12 D BF 100 BF 100 BF 100 456 590 24 208 190 201 13 EBF 100 BF 100 BF 100 450 568 18 183 171 178 14 F BF 100 BF 100 BF 100396 495 18 170 160 165 15 G BF + M  85 BF 100 BF 100 520 672 14 235 211214 16 H BF 100 BF 100 BF 100 537 667 14 222 209 212 17 I BF 100 BF 100BF 100 550 690 14 226 213 216 18 J BF + F  90 BF + F  90 BF + F  80 408515 18 176 156 170 19 K BF + M  86 BF + M  92 BF + M  95 442 571 16 199179 185 20 L BF 100 BF + F  85 BF + F  75 450 568 17 184 160 182 21 MBF + M  85 BF + M  90 BF + M  92 512 639 15 223 193 203 22 N BF 100 BF100 BF 100 460 580 26 197 182 190 23 O BF 100 BF 100 BF 100 483 606 25206 194 200 24 P BF 100 BF 100 BF 100 518 630 24 213 199 206 25 Q BF + M 92 BF 100 BF 100 551 684 15 230 207 214 26 R BF 100 BF 100 BF + F 90316 421 23 138 130 135 Characteristics after Thin Form- heat treatmentsteel Uniformity Ability Tensile characteristics Toughness panel SteelΔHV/HV_(mean) Minimum bend YS TS El vTrs Nos. Nos. (%) radius** (mm)(MPa) (MPa) (%) (° C.) Remarks  1 A  6 0.5 t 898 1005 21 <−100   IE  2 B 5 0.5 t 932 1040 20 <−100   IE  3 C  7 0.5 t 1004 1120 19 −100 IE  4 C15 2.0 t 1077 1096 13 −100 CE  5 C 18 2.0 t 989 1100 13 −100 CE  6 C 161.5 t 991 1104 14 −100 CE  7 C 16 2.0 t 987 1104 12 −100 CE  8 C 12 1.5t 990 1096 11 −100 CE  9 C 13 1.5 t 1001 1104 12 −100 CE 10 C 11 1.5 t1006 1118 12 −100 CE 11 C 18 1.5 t 1002 1114 12 −100 CE 12 D  9 0.5 t1014 1180 18  −80 IE 13 E  7 1.0 t 1003 1112 11 −100 CE 14 F  6 0.5 t770  965 14 −100 CE 15 G 11 1.5 t 1017 1104 7  −40 CE 16 H  6 1.5 t 9961100 10  −20 CE 17 I  6 1.5 t 1002 1112 8  −40 CE 18 J 12 2.0 t 830  94514  −50 CE 19 K 11 1.5 t 986 1096 13  −40 CE 20 L 13 2.0 t 822  917 14 −20 CE 21 M 15 2.0 t 992 1100 13 −100 CE 22 N  8 0.5 t 999 1104 17 −100IE 23 O  6 0.5 t 1020 1128 17 −100 IE 24 P  7 0.5 t 1035 1156 16 −100 IE25 Q 11 1.5 t 1094 1213 8  −50 CE 26 R  6 1.0 t 860  952 16 −100 CE *F:ferrite (massive form), B: bainite, M: martensite, BF: bainitic ferrite**t: sheet thickness of a steel sheet (mm) IE: Invention Example, CE:Comparative Example

In all Invention Examples, a bainitic ferrite phase having an area ratioof 95% or more (bainitic ferrite single phase) is uniformly formed in asheet thickness direction, whereby there is provided a high strength hotrolled thick steel sheet with excellent formability that has a tensilestrength of 440 MPa or more and an elongation of 20% or more; that isexcellent in uniformity because the deviation of hardness ΔHV alongthickness is within 10% from the average hardness value (average)HV_(mean); and that is excellent in bending workability with a minimumbend radius of 0.5t or less. Furthermore, high strength with a tensilestrength of 980 MPa or more, high ductility with an elongation of 15% ormore, and high toughness with a vTrs of −60° C. or less can be achievedby performing quenching and tempering treatment. In contrast, in theComparative Examples, a uniform bainitic ferrite phase is not formed and“strength or ductility” or “strength and ductility” do not reach theabove-described desired values. Furthermore, the deviation of hardnessΔHV along thickness becomes large and the uniformity in the sheetthickness direction is decreased. In addition, one or more of strength,ductility, and toughness after quenching and tempering treatment do notreach the above-described desired values, which provides a hot rolledsteel sheet that lacks any of strength, ductility, and toughness afterquenching and tempering treatment.

1. A high strength hot rolled thick steel sheet with a sheet thicknessof 6 mm or more and 12 mm or less that is excellent in strength andtoughness after heat treatment, the hot rolled thick steel sheetcomprising a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%,Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%,N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by masswith the balance Fe and incidental impurities; and a bainitic ferritephase having an area ratio of 95% or more, wherein a deviation ofhardness along thickness is within 10% from an average; and having atensile strength of 440 to 640 MN and an elongation of 20% or more(gauge length GL: 50 mm).
 2. A method for manufacturing a high strengthhot rolled thick steel sheet that is excellent in strength and toughnessafter heat treatment, the method comprising: hot-rolling a steelmaterial at a finisher delivery temperature of 820 to 880° C. in finishrolling to obtain a hot rolled steel sheet having a sheet thickness of 6mm or more and 12 mm or less, the steel material having a compositionincluding C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03%or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti:0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe andincidental impurities; cooling the hot rolled steel sheet at a coolingrate of 15 to 50° C./s on a surface temperature basis until a surfacetemperature reaches a temperature range of 550 to 650° C.; and coilingthe hot rolled steel sheet in the temperature range, such that the steelsheet has a deviation of hardness along thickness within 10% from anaverage, a tensile strength of 440 to 640 MPa and an elongation of 20%or more (gauge length GL: 50 mm).